1. Field of the Invention
The present invention generally relates to a thin film magnetic memory device. More particularly, the present invention relates to a random access memory (RAM) including memory cells having a magnetic tunnel junction (MTJ).
2. Description of the Background Art
An MRAM (Magnetic Random Access Memory) device has attracted attention as a memory device capable of non-volatile data storage with low power consumption. The MRAM device is a memory device capable of non-volatile data storage using a plurality of thin film magnetic elements formed in a semiconductor integrated circuit and also capable of random access to each thin film magnetic element.
In particular, recent announcement shows that the use of thin film magnetic elements having a magnetic tunnel junction (MTJ) as memory cells significantly improves performance of the MRAM device. The MRAM device including memory cells having a magnetic tunnel junction is disclosed in technical documents such as xe2x80x9cA 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cellxe2x80x9d, ISSCC Digest of Technical Papers, TA7.2, February 2000, and xe2x80x9cNonvolatile RAM based on Magnetic Tunnel Junction Elementsxe2x80x9d, ISSCC Digest of Technical Papers, TA7.3, February 2000.
FIG. 17 schematically shows the structure of a memory cell having a magnetic tunnel junction (hereinafter, sometimes simply referred to as xe2x80x9cMTJ memory cellxe2x80x9d).
Referring to FIG. 17, the MTJ memory cell includes a tunneling magneto-resistance element TMR having its electric resistance varying according to a magnetically written storage data level, and an access transistor ATR. Access transistor ATR is connected in series with tunneling magneto-resistance element TMR between a write bit line WBL and a read bit line RBL. A field effect transistor formed on a semiconductor substrate is typically used as access transistor ATR.
A write bit line WBL, a write digit line WDL, a word line WL and a read bit line RBL are provided for the MTJ memory cell. Write bit line WBL and write digit line WDL pass data write currents of different directions therethrough in data write operation, respectively. Word line WL is used to conduct data read operation. Read bit line RBL receives a data read current. In data read operation, tunneling magneto-resistance element TMR is electrically coupled between write bit line WBL having a ground voltage GND and read bit line RBL in response to turning-ON of access transistor ATR.
FIG. 18 is a conceptual diagram illustrating data write operation to the MTJ memory cell.
Referring to FIG. 18, tunneling magneto-resistance element TMR has a ferromagnetic material layer FL having a fixed magnetization direction (hereinafter, sometimes simply referred to as xe2x80x9cfixed magnetic layerxe2x80x9d), and a ferromagnetic material layer VL that is magnetized in the direction according to an external magnetic field (hereinafter, sometimes simply referred to as xe2x80x9cfree magnetic layerxe2x80x9d). A tunneling barrier (tunneling film) TB of an insulator film is interposed between fixed magnetic layer FL and free magnetic layer VL. Free magnetic layer VL is magnetized either in the direction that is the same as or opposite (antiparallel) to that of fixed magnetic layer FL according to a write data level. Fixed magnetic layer FL, tunneling barrier TB and free magnetic layer VL form a magnetic tunnel junction.
The electric resistance of tunneling magneto-resistance element TMR varies according to the relation between the respective magnetization directions of fixed magnetic layer FL and free magnetic layer VL. More specifically, the electric resistance of tunneling magneto-resistance element TMR is minimized (Rmin) when fixed magnetic layer FL and free magnetic layer VL have parallel magnetization directions, and is maximized (Rmax) when they have opposite (antiparallel) magnetization directions.
In data write operation, word line WL is inactivated and access transistor ATR is turned OFF. In this state, a data write current for magnetizing free magnetic layer VL is applied to each of write bit line WBL and write digit line WDL in a direction according to the write data level.
FIG. 19 is a conceptual diagram illustrating the relation between the data write current and the magnetization direction of the tunneling magneto-resistance element in the data write operation.
Referring to FIG. 19, the abscissa H(EA) indicates a magnetic field that is applied to free magnetic layer VL of tunneling magneto-resistance element TMR in the easy-axis (EA) direction. The ordinate H(HA) indicates a magnetic field that is applied to free magnetic layer VL in the hard-axis (HA) direction. Magnetic fields H(EA), H(HA) respectively correspond to two magnetic fields produced by the currents flowing through write bit line WBL and write digit line WDL.
In the MTJ memory cell, fixed magnetic layer FL is magnetized in the fixed direction along the easy axis of free magnetic layer VL. Free magnetic layer VL is magnetized either in the direction parallel or antiparallel (opposite) to that of fixed magnetic layer FL along the easy axis according to the storage data level (xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d). The MTJ memory cell is thus capable of storing 1-bit data (xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d) according to the two magnetization directions of free magnetic layer VL.
The magnetization direction of free magnetic layer VL can be rewritten only when the sum of the applied magnetic fields H(EA) and H(HA) reaches the region outside the asteroid characteristic line shown in the figure. In other words, the magnetization direction of free magnetic layer VL will not change if an applied data write magnetic field corresponds to the region inside the asteroid characteristic line.
As shown by the asteroid characteristic line, applying a magnetic field of the hard-axis direction to free magnetic layer VL enables reduction in a magnetization threshold value required to switch the magnetization direction along the easy axis.
When the operation point of the data write operation is designed as in the example of FIG. 19, a data write magnetic field of the easy-axis direction is designed to have a strength HWR in the MTJ memory cell to be written. In other words, the data write current to be applied to write bit line WBL or write digit line WDL is designed to produce the data write magnetic field HWR. In general, data write magnetic field HWR is defined by the sum of a switching magnetic field HSW required to switch the magnetization direction and a margin xcex94H. Data write magnetic field HWR is thus defined by HWR=HSW+xcex94H.
In order to rewrite the storage data of the MTJ memory cell, that is, the magnetization direction of tunneling magneto-resistance element TMR, a data write current of at least a prescribed level must be applied to both write digit line WDL and write bit line WBL. Free magnetic layer VL in tunneling magneto-resistance element TMR is thus magnetized in the direction parallel or opposite (antiparallel) to that of fixed magnetic layer FL according to the direction of the data write magnetic field along the easy axis (EA). The magnetization direction written to tunneling magneto-resistance element TMR, i.e., the storage data of the MTJ memory cell, is held in a non-volatile manner until another data write operation is conducted.
FIG. 20 is a conceptual diagram illustrating data read operation from the MTJ memory cell.
Referring to FIG. 20, in data read operation, access transistor ATR is turned ON in response to activation of word line WL. Write bit line WBL is set to ground voltage GND. As a result, tunneling magneto-resistance element TMR pulled down to ground voltage GND is electrically coupled to read bit line RBL.
If read bit line RBL is then pulled up to a prescribed voltage, a memory cell current Icell according to the electric resistance of tunneling magneto-resistance element TMR, that is, the storage data level of the MTJ memory cell, flows through a current path including read bit line RBL and tunneling magneto-resistance element TMR. For example, the storage data can be read from the MTJ memory cell based on comparison between memory cell current I cell and a prescribed reference current.
The electric resistance of tunneling magneto-resistance element TMR thus varies according to the magnetization direction that is rewritable by an applied data write magnetic field. Accordingly, non-volatile data storage can be realized by using electric resistances Rmax and Rmin of tunneling magneto-resistance element TMR as the respective storage data levels (xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d).
The MRAM device stores data by using the difference between junction resistances (xcex94R=Rmaxxe2x88x92Rmin) that corresponds to the difference in storage data level in tunneling magneto-resistance element TMR.
In general, the MRAM device includes reference cells for producing a reference current to be compared with a memory cell current I cell, in addition to the normal MTJ memory cells for storing data. The reference cells are designed to produce a reference current that is equal to an intermediate value of the two memory cell currents I cell respectively corresponding to the two electric resistances Rmax and Rmin of the MTJ memory cell. Basically, these reference cells are designed and fabricated in the same manner as that of the normal MTJ memory cells. In other words, the reference cells also include a tunneling magneto-resistance element TMR having a magnetic tunnel junction.
However, a current passing through tunneling magneto-resistance element TMR is significantly affected by the thickness of an insulating film used as a tunneling film. Accordingly, if the normal MTJ memory cell and the reference cell have any difference in thickness of the tunneling film, it would be difficult to set the reference current to the level that allows the small current difference to be sensed. This may possibly reduce the accuracy of the data read operation.
In particular, in a common MTJ memory cell, the resistance difference xcex94R produced according to the storage data level is not so large. Typically, electric resistance Rmin is at most about several tens of percents of Rmax. Memory cell current Icell therefore varies at most on the order of microamperes (xcexcA: 10xe2x88x926 A) according to the storage data level. Accordingly, the respective tunneling films of the normal MTJ memory cell and the reference cell must be formed with an accurate thickness.
However, such a strict manufacturing process regarding accuracy of the thickness of the tunneling film may reduce the manufacturing yield, thereby possibly increasing the manufacturing costs. Accordingly, there is a demand for the MRAM device capable of accurately conducting data read operation based on the resistance difference xcex94R in the MTJ memory cell without requiring a strict manufacturing process.
It is an object of the present invention to provide a thin film magnetic memory device capable of conducting accurate data read operation without using a reference cell.
In summary, according to the present invention, a thin film magnetic memory device includes a plurality of memory cells, a data line, a read current supply circuit, and a data read circuit.
Each memory cell is magnetized in a direction according to storage data thereof having either a first or second level. Each memory cell has an electric resistance according to a magnetization direction. The data line is electrically coupled to a selected memory cell for a prescribed period in data read operation. The read current supply circuit supplies a data read current to the data line in order to produce a voltage according to the electric resistance of the selected memory cell onto the data line. The data read circuit produces read data corresponding to the storage data of the selected memory cell, based on a voltage on the data line electrically coupled to the selected memory cell in a first state and a voltage on the data line electrically coupled to the selected memory cell in a second state. The first state is a state where the selected memory cell has a same magnetization direction as before the data read operation. The second stage is a state after the selected memory cell is subjected to a prescribed magnetic field.
The above thin film magnetic memory device is capable of conducting data read operation by merely accessing the selected memory cell without using a reference cell. In other words, the above thin film magnetic memory device is capable of conducting data read operation based on the comparison between voltages obtained by the same data read path including the same memory cell, data line and the like. Accordingly, the data read operation can be conducted with improved accuracy without being subjected to the influences such as an offset resulting from manufacturing variation of the circuits included in the data read path.
Preferably, the thin film magnetic memory device further includes a write control circuit for writing the storage data to one of the plurality of memory cells. The selected memory cell changes from the first state to the second state when the write control circuit writes storage data of a prescribed level thereto in a single data read operation. The write control circuit rewrites storage data having a same level as that of the produced read data to the selected memory cell in the single data read operation.
The above thin film magnetic memory device is capable of conducting data read operation from the selected memory cell without using a reference cell. In a single data read operation, the above thin film magnetic memory device reads storage data from the selected memory cell before and after writing data of a prescribed level thereto, and produces read data based on the comparison therebetween. Moreover, the read data is rewritten to the selected memory cell in the single data read operation. This enables the selected memory cell to restore to the state before the data read operation.
More preferably, the write control circuit skips the rewrite operation when the storage data in the selected memory cell has a same level as that of the produced read data before the rewrite operation.
As a result, an unnecessary rewrite operation is omitted, enabling reduction in current consumption of the data read operation.
Preferably, a single data read operation includes an initial read operation for obtaining a voltage on the data line electrically coupled to the selected memory cell in the first state, a first prescribed write operation for writing data having a prescribed level to the selected memory cell, a first prescribed read operation for obtaining a voltage on the data line electrically coupled to the selected memory cell after the first prescribed write operation, a read data determining operation for determining the read data after the first prescribed read operation based on the respective voltages on the data line obtained in the initial read operation and the first prescribed read operation, and a rewrite operation for rewriting the storage data having a same level as that of the determined read data to the selected memory cell after the read data determining operation.
The above thin film magnetic memory device is capable of conducting data read operation from the selected memory cell without using a reference cell. In a single data read operation, the above thin film magnetic memory device reads storage data from the selected memory cell before and after writing data of a prescribed level thereto, and produces read data based on the comparison therebetween. Moreover, the read data is rewritten to the selected memory cell in the single data read operation. This enables the selected memory cell to restore to the state before the data read operation.
Preferably, each memory cell is magnetized along an easy-axis direction according to the storage data. The thin film magnetic memory device further includes a bias magnetic field applying portion for applying a prescribed bias magnetic field to the selected memory cell. The prescribed bias magnetic field has a component along a hard-axis direction. The selected memory cell changes from the first state to the second state when the bias magnetic field is applied thereto.
More preferably, the data read circuit includes a sense amplifier for amplifying a voltage difference between the data line electrically coupled to the selected memory cell and a first node, a voltage holding portion for holding a voltage on the first node, a switch circuit for connecting an output node of the sense amplifier to the first node in the first state, and disconnecting the output node of the sense amplifier from the first node in the second state, and a read data producing circuit for producing the read data according to a voltage on the output node in the second state.
Accordingly, a data line voltage corresponding to the storage data of the selected memory cell can be obtained by using negative feedback of the sense amplifier. This suppresses an offset produced in the sense amplifier, enabling further improvement in accuracy of the data read operation.